1. Field of the Invention
The present invention relates to a fingerprint image input apparatus, and, more particularly, to a fingerprint image input apparatus capable of directly inputting a fingerprint image from a finger without using ink.
2. Description of the Related Art
Various fingerprint image input apparatuses capable of directly inputting a fingerprint image from a finger without using ink have been proposed. For example, Unexamined Japanese Patent Publication No. Sho 58-201178 discloses a fingerprint image input apparatus which can acquire a fingerprint image of a finger pressed against a transparent body by using an optical system like a prism and a CCD camera. Hereinafter, this apparatus is called the first prior art.
FIG. 1 is an exemplary diagram showing a fingerprint image input apparatus according to the first prior art. This apparatus starts imaging upon detection of the pressure of a finger 50. When the finger 50 is pressed against a prism 51, the prism 51 presses pressure-sensitive sections 54 via associated springs 52 and metal fittings 53. Each pressure-sensitive section 54 sends a fingerprint detection signal to an AND gate 55 only when the applied pressure becomes equal to or greater than a given level or a given range. When all the pressure-sensitive sections 54 (for example, four pressure-sensitive sections) send fingerprint detection signals to the AND gate 55, an ON signal from the AND gate 55 is sent to a controller 58 which in turn drives a light source 56 and an fingerprint image sensor 57. A fingerprint image picked up in this manner is sent to another processing section from an interface 59 under the control of the controller 58.
Unexamined Japanese Patent Publication No. Sho 64-13677 discloses another type of fingerprint image input apparatus whose imaged fingerprint images have no distortion. This apparatus is hereinafter called the second prior art.
FIG. 2 is an exemplary cross-sectional view of a fingerprint image input apparatus according to the second prior art. In the second prior art, a curved glass body 61 whose inner wall and outer wall are concentrical is used as a transparent body against which a finger 60 is to be pressed. A one-dimensional image sensor 62 and an illuminator 63 for illumination over the imaging area of the one-dimensional image sensor 62 are fixed with such a positional relation as to satisfy the condition for full reflection at the inner wall of the curved glass body 61. The one-dimensional image sensor 62 has an optical fiber lens 64 which is so fixed as to focus on the inner wall of the curved glass body 61.
The curved glass body 61 is provided with a light-emitting diode 65 and a phototransistor 66 which are so arranged to face each other. When the finger 60 is placed on the inner wall of the curved glass body 61, light from the light-emitting diode 65 is blocked and the phototransistor 66 detects a change in light to thereby detect the presence of the finger 60.
Then, the one-dimensional image sensor 62 and the illuminator 63 move along the outer wall of the curved glass body 61 while satisfying the condition for full reflection at the inner wall of the curved glass body 61 to thereby pick up a fingerprint image.
Unexamined Japanese Patent Publication No. Hei 1-180685 discloses a fingerprint image input apparatus which has an improved finger detecting section. This apparatus is hereinafter called the third prior art.
FIG. 3 is a perspective view of a fingerprint image input apparatus according to the third prior art. In the third prior art, a plurality of transparent electrodes 71 and 72 are arranged in a comb shape on the inner wall of a curved glass body 70 so as to face each other without any contact. When a finger is placed on the inner wall of the curved glass body 70, sweat produced by the perspiration of the skin short-circuits the transparent electrodes 71 and 72. The presence of the finger is detected by this short-circuiting so that imaging can start.
Those conventional fingerprint image input apparatuses detect, as a fingerprint image, the difference between the amount of the reflected light from the portion of the transparent body with which a finger is in contact and that from the portion of the transparent body which is not touched with a finger, by using an optical system like a prism and a one-dimensional CCD sensor.
Those first to third prior arts use a combination of an image sensor, such as a television camera or one-dimensional CCD sensor, and an optical part such as a prism or a curved transparent body. Those apparatuses therefore become large and have large-scale structures.
Unexamined Japanese Patent Publication No. Sho 60-235456 discloses a close-contact type image sensor which has a high reliability and high resolution. This image sensor has at least one sensor having a multilayer structure. The image sensor is realized by forming a photodiode, made of an amorphous silicon (a-Si) as a base substance, on an insulating substrate, and forming a blocking diode, made of an amorphous silicon as a base substance, on this photodiode via a conductive amorphous silicon.
A similar sensor comprising amorphous silicon thin-film transistor, photodiodes having a p-i-n junction is disclosed in MRS Bulletin, Vol. 16, No. 11, p. 70-76 (1992).
U.S. Pat. No. 5,446,290 discloses a thin and compact finger print image input apparatus. FIG. 4 is a perspective view showing the fingerprint image input apparatus to which such an image sensor using amorphous silicon is adapted. This image sensor is hereinafter called the fourth prior art. As shown in FIG. 4, this fingerprint image input apparatus 5 comprises a plane light source 1, a two-dimensional image sensor 2 and an optical part 3.
FIG. 5 is a circuit diagram showing the structure of the image sensor 2 in FIG. 4. As shown in FIG. 5, the image sensor 2 comprises a plurality of pixels 10 arranged in a matrix form, a scanning circuit 16 for outputting scan signals to the individual pixels 10, scan lines 13 for transferring scan signals, bias lines 15 and 15a for applying a bias voltage to the individual pixels 10, data lines 14 for transferring the output signals of the individual pixels 10 and a detecting circuit 17 for detecting data from the data lines 14.
Each pixel 10 is constituted of a light-receiving element 11, made of a photodiode formed by a thin film of a-Si or the like, and a switch element 12, made of a thin-film transistor formed by a thin film of a-Si or the like.
In the thus constituted fingerprint image input apparatus 5, light emitted from the plane light source 1 passes through the clearances between the pixels 10 formed on the image sensor 2 and reaches the optical part 3. The path of the light is defined in such a way that the light obliquely hits a finger (not shown) placed on the optical part 3. Then, the light is reflected at the finger and is guided to the light-receiving elements 11 of the pixels 10 again through the optical part 3. Prior to detecting the reflected light, the individual light-receiving elements 11 are charged by the application of a reverse bias voltage via the bias lines 15 and 15a, and the individual switch elements 12 are set off. Therefore, the light-receiving elements 11 stand by, ready for the detection of light.
When the reflected light from the finger is absorbed by the light-receiving elements 11, the amount of the electric charges accumulated in the light-receiving elements 11 decreases in accordance with the charges produced by the reflected light. Then, the scanning circuit 16 simultaneously enables the switch elements 12 of an arbitrary row. Consequently, charges flow through the data lines 14 to those of the light-receiving elements 11 connected to the enabled switch elements 12 which are discharged by the light illumination. As the current is detected by the detecting circuit 17, the light-receiving elements 11 which are irradiated with light can be detected. By performing a similar operation for all the rows, a two-dimensional image as the distribution of the reflected light from the finger can be read out.
If the presence of a finger on the fingerprint image input apparatus 5 is automatically detected in the fourth embodiment, a stable image can be obtained. As the image input is automatically initiated, the operability is improved. A touchpanel input apparatus capable of detecting the contact of a finger by using electrostatic capacitors is disclosed in Unexamined Japanese Patent Publication No. Hei 5-335925.
FIG. 6 is an exemplary diagram showing the structure of the conventional touchpanel input apparatus. A plurality of electrodes 104a, 104b and 104c are formed on a glass substrate 100, with a protective film 103 formed on the top of those electrodes 104a, 104b and 104c. The individual electrodes 104a, 104b and 104c are connected via an impedance element 106 to a voltage generator 105.
When a finger 101 touches the protective film 103 directly above the electrode 104b of the thus constituted touchpanel input apparatus, for example, a parasitic capacitor Cm of the human body and a parasitic capacitor Cn between the selected electrode 104b and the finger 101 are connected in series, and these capacitors and a fixed capacitor (or parasitic capacitor) Cp of the selected electrode 104b are connected in parallel so that the combined capacitance increases. Therefore, a voltage V.sub.out across both ends of the fixed capacitor Cp is influenced by the presence of a finger. As a change in the terminal voltage V.sub.out is measured, the touched electrode can be identified.
To incorporate any of the finger detecting means of the first to third prior arts (the use of the electrostatic capacitor, the pressure detection, the use of light reflection and the detection of electric short-circuiting) into the image input apparatus of the fourth prior art, however, those detecting means should be provided at finger-touching portions (optical part 3). This design deteriorates the close contactability between the optical part 3 and a finger, thus degrading the image quality.
While the pressure detecting means is attachable to the back of the sensor, this design should sacrifice the flatness and compactness of the apparatus.
If finger detecting means is attached to each image sensor, the manufacturing cost is inevitably increased.